In many mechanical environments, there will be present both oil based and water based compounds. Many machines use hydraulic power incorporating a high pressure hydraulic fluid, an oil, in a sealed environment. However, seals age and leakage of hydraulic fluid can occur to contaminate metalworking fluid.
Mechanical processes often need a lubricating compound to prevent friction between moving parts. An example of such a system is industrial metal-working machine tools wherein the work piece and tooling are positioned and moved on flat bearings generally referred to as slideways. These surfaces require a lubricant—the slideway lubricant—to reduce the metal/metal contact therefore eliminating friction and ‘stick-slip’ motion, and prevent wear on the slideways. This is essential to maintain the potentially very expensive machine tool in good condition and to ensure machining accuracy. As most machining processes also utilise a flood applied, water-oil emulsion based metalworking fluid to cool and lubricate the working zone the slideway lubricant is constantly washed off the surfaces of the slides. To combat this most machine tools automatically dispense oil onto the slides on a continuous or metered basis. Given the very low cost of the slideway lubricant this total loss system is the most cost effective method of ensuring that the performance of the machine is not compromised. The lost lubricant, tramp oil, is washed with the metalworking fluid into a sump, from which the metalworking fluid is recycled.
Whilst such methods ensure that the machine stays in good condition it has the opposite effect on the metalworking fluids. Being oil-water emulsions, metalworking fluids provide an ideal environment for microbial growth. Although great efforts are made when formulating metalworking fluids to exclude obvious sources of nutrients and to ensure that the fluids are as resistant as possible to microbial growth this is largely negated if contaminated by the slideway lubricant. Since slideway lubricants are generally based on refined mineral oils and contain elements such as sulphur and phosphorus (as anti-wear and anti-corrosion agents), these provide ideal nutrients for micro-organisms. It is accepted throughout the metalworking industry that one of the main causes of failure for metalworking fluids is microbial growth promoted by excessive contamination with slideway lubricants. In systems with low agitation the slideway oil can separate from the emulsion, sealing it from the air. In such anaerobic conditions microbial growth in fluids produces by-products of respiration, such as Hydrogen Sulphide which causes an unpleasant odour and makes the system unpleasant to work with.
Hand applied tapping fluids, may be used for tapping, reaming and drilling operations, and when used on machine tools with water mix, flood applied cutting fluids can also contaminate the cutting fluid.
Several attempts have been made to overcome this problem, which include:                i) Use of synthetic, biologically-hard base fluids instead of mineral oil as the base for slideway lubricants. While this may reduce the overall level of nutrient for the microbes the key sulphur and phosphorus agent agents are still present. Synthetic base fluids are also prohibitively expensive for ‘total loss’ systems.        ii) Use of the metalworking fluid concentrate as the slideway lubricant. Although this overcomes the contamination problem the primary function as a slideway lubricant is compromised. The ease with which these products can be washed away can leave slideways ‘dry’ resulting in high friction and poor control of the tooling/work piece. The emulsification of this additional concentrate can also lead to the cutting fluid strength increasing to excessive levels.        iii) Use of oil skimmers/separators. These can vary in both price and performance. The simplest and cheapest types are either belt or disc skimmers that are immersed directly into machine sumps and pick up free oil from the surface of metalworking fluids. These are limited in their performance due to the fact that the oil will only separate from the fluid when there is no agitation (i.e. in ‘dead’ areas of the sump) or when the fluid is saturated with oil. At the other end of the scale are stand alone machines that extract fluid from machine sumps and remove any oil contamination. These can either be static, dedicated to a single machine, or mobile to service multiple machines on a rota basis. Although they are effective at removing the contamination from the fluids the cost of these units can be tens of thousands of pounds each. This can mean a significant capital investment even for moderately sized engineering shops.        iv) use of an anti-microbial agent in the cutting fluid, anti-nicrobials are dangerous chemicals, especially in concentrated form as an agent, and the anti-microbial needs to be added in correct dosages to maintain function.        